The present invention relates to a cover layer for a metal anode for electrochemical processes, particularly those processes carried out in a highly corrosive environment such as the electrochemical production of chlorine and caustic soda.
Platinum, platinum metals and alloys thereof have been known for a long time as electrode materials. For example, the first horizontal mercury cells were equipped with anodes of platinum and platinum-iridium for the electrolytic production of chlorine and caustic soda. The rather high investment costs for equipping the anodes with platinum wire and the considerable corrosion rates of the valuable precious metal-- the specific platinum loss even at relatively low current densities reached from 0.3 to 0.6 grams of platinum per ton of produced chlorine -- soon made it necessary to adopt the more economical graphite anodes.
The idea of coating a non-precious base metal; such as copper, iron, etc.; with platinum in order to obtain an anode material at a reasonable price is also very old. In chlorine-alkali electrolysis these platinum-plated metal anodes rather quickly succumbed to the corrosive influences of the cell media.
Similarly, metals such as titanium, tantalum, zirconium, niobium, and alloys thereof have been used as anode basis metals. These have become known as "valve" metals because of their tendency to passivate rapidly by the formation of a tight oxide cover layer, which layer effects a rectifier effect on current passing therethrough. The valve metal anodes have been covered with a cover layer of platinum or platinum alloys but these also have been found unsuitable for use in the chlorine alkali industry. The expensive plating of precious metal does not withstand for any length of time the various intensive stresses; whether of mechanical, electrical, chemical or electrochemical nature; which prevail in modern giant cells. The effectiveness of the platinum metal layer which, for reasons of cost is kept rather thin, soon decreases. This, on one hand, leads to continuing increases in the voltage and, on the other hand, makes a frequent exchange of the anode necessary. In the widely used horizontal mercury cells the danger of short-circuiting is particularly great and with the tendency of platinum to form an amalgam the danger exists that the entire metal anode equipment will suddenly become inactive.
It is therefore the principal object of this invention to provide an improved cover layer for an electrolytic anode basis metal where said cover layer is economical; is thermally stable; is not soluble in standard solvents; is stable in electrolytic baths, particularly those containing salt brines and mercury; has a relatively low anodic overvoltage for chlorine evolution; has a relatively high anodic overvoltage for oxygen evolution; and is resistant to various intensive stresses of a mechanical, chemical, and electrochemical nature.